...so as not to gainsay Darwin does sound rather like treating the book as The Book... Holy Writ.

That's a pretty silly remark. Darwin demarcated natural selection, for the purpose of his theory, as the non-random survival of heritable factors within species. You draw up one of the greatest of all scientific theories not by being sloppy but by defining your frames of reference carefully. I'll leave it to the "social Darwinists" or eugenicists, far lesser men than Darwin, to tendentiously redefine his terms, thank you.